Hi Ken: I'm sympathetic to that point of view.Except if we only defined winnable as having placed 2nd last time, then several seats actually won by the NDP in recent elections would not have been "winnable" by that definition.
In my lifetime, the NDP has held Kenora (1984), Desnethe-Missinippi-Churchill River (1988), and Nunavut (1980) federally, and Guelph provincially (in the 1990 provincial).
If the NDP is on an uptick, then they could be pretty good ridings.
It will be interesting to test that theory in the upcoming Guelph by-election, I think.